The present invention relates to a device for transmitting the position of an object that is contained within a closed vessel and has no direct connection with the device, and more particularly concerns a device that is digital in design, solid-state in construction, low in cost, versatile in application, extremely low in power consumption, and simple in installation and that transmits an output signal that has the qualities of high precision, drift-free accuracy, excellent noise immunity, and moderate resolution.
At the present time there are a number of industrial processes that utilize the general combination of a piston or float contained within a closed vessel. The vessel is used to contain the process fluids and separate them from the ambient environment while the piston or float is used to indicate the current stage of the process. Some general examples of these processes include rotameter-type flow meters, pressure indicators, storage vessels for hazardous or flammble chemicals, containment systems for excess pressure, and any process isolated for it's hazards.
The present invention was originally developed to meet the needs of the particular process known as continuous composite sampling within the specific Oil and Gas Industry but is not limited in application to that particular process or to that particular industry.
Continuous composite sampling is a measurement system that answers the question "what is the composition and therefore value per unit of flow" of the petroleum product that was received, produced, or transported, at a specific location, during a specific period of time? It is related to general measurement systems that include volume and mass flowmeters, density meters, calorimeters, gravitometers, process chromatographs, and viscocity meters.
Two other popular schemes for analyzing the quality or value of petroleum fluids include direct process analysis and batch analysis. All three schemes share the steps of store, extract, mix and analyze but each scheme performs the steps in a different order.
In batch sampling the entire amount of the fluid that is processed during the sampling period is stored and mixed before a sample is extracted and analyzed and before the fluid is available for further processing. The fact that the fluid is unavailable for further processing until the end of the period causes the batch sampling method to be impractical for processes involving large quantities of fluid and long sampling periods; the storage vessel would be enormous and the flow would be interrupted by the storage step. Trying to batch sample a continuous process, such as large pipeline flow is inherently impractical. Note however, that batch sampling is the most straightforward in concept and trouble-free in practice and serves as a standard for evaluation of the other schemes.
In process analysis thousands of samples are extracted and analyzed during any sampling period. The result of each analysis is electronically mixed with other current process parameters such as flow, temperature and pressure, and the integrated data is stored. The integrated data can be retrieved on demand and totalled for any time period without destroying the original data. Process analysis can be very accurate but uses complicated equipment that is expensive to purchase, difficult to maintain, and complicated to operate and must normally be housed in an air conditioned building. Therefore this scheme is impractical for remote, unmanned locations with a small power supply and is difficult to justify unless the economics of the process are favored by detailed, up-to-date analytical information.
Composite sampling is also called continuous sampling, continuous composite sampling, representative sampling, proportional sampling, or simply gas sampling, and is the scheme for which the present invention is developed.
In composite sampling, a very small but controlled quantity of fluid is extracted at frequent intervals from the process flow and stored in the sample vessel during a sampling period that is typically a week or a month. At the end of the sample period, the sample vessel is removed to the laboratory, the fluid is mixed within the vessel to stabilize the composition, and then the contents of the vessel are analyzed. If the sampling is accurate, the results of this single, end-of-the-period analysis are representative of the total amount of fluid that was processed during the period. An accurate sample is composed of thousands of individual extractions, where the quantity of each extraction and the frequency of extractions is precisely controlled by either a process flow signal or a timer.
In theory, the sample vessel is a small replica of the enormous storage vessel from the batch sampling scheme and each extraction is a small replica of the fluid that is processed between extractions. Note that typically the volume of the sample vessel is less than 1 liter, while the volume of the total process flow is greater than 1 million liters.
The essential components of a composite sampler are the controller, the sample extractor and the sample vessel. The present invention has been developed to transmit the level of fullness of the particular type of sample vessel that is generally referred to as a "constant pressure cylinder".
In it's simplest form the sample vessel consists of a small, high pressure cylinder with a valve at one or both ends and a pressure gauge or transmitter to indicate the state of fullness. This type of sample vessel is referred to as a "constant volume cylinder" and is typically used when the fluid to be sampled is primarily in the gas phase so that the pressure reading is a reliable indication of how much fluid has been sampled.
By contrast, the constant pressure cylinder is a complex device, consisting of a precision bored and polished cylinder that has been clamped between two precision fitted endcaps and that has been separated into two chambers by a ringed piston that will not allow fluid to pass between the chambers but will slide freely within the cylinder, varying the volume of each chamber until their pressures are equal. There is usually a valve, a pressure relief device and a pressure gauge at each end.
The constant pressure cylinder can be used to sample fluids in either the gas or liquid phase and it is the position of the piston within the cylinder, and not the pressure, that indicates the state of fullness. The position of the piston is a direct volume indication of how much fluid is currently stored in the sample vessel.
It is difficult to determine the position of this piston because, by function and design, the piston must be surrounded on the sides by the steel walls of the cylinder and at the ends by the pressurized petroleum fluids and further because the interior of the cylinder should be practically free of devices or pressure seals that would compromise the integrity of the sampled fluid. It is this relative isolation of the piston and the strict requirements of sample integrity that favors the use of a non-contact position transducer and causes the commercially available, low cost position transducers of the prior art to be unreliable or hazardous.
To date, the most common and reliable solution has been to fit the end of the piston with a set of three ring magnets, and to attach a linear array of magnetic flags along the outside of the cylinder; as the piston moves along the cylinder the advancing magnetic field causes the flags to turn over, exposing a side of a different color.
Another common indicator, closely related to the one described above, utilizes rod magnets that are embedded in the piston and the magnetic flag array is replaced by a brightly colored magnetic tracker contained within a clear plastic tube. The colored tracker is magnetically coupled to the magnets in the piston and therefore follows it's movement.
The major drawback to both these types of indicators is that they are only indicators and not transmitters; a human operator must go out to the sampling location and visually inspect the indicator in order to determine the position of the piston and therefore the current state of fullness of the sample vessel. In order to ascertain the operability of the composite sampler the operator would be required to make a number of observations over the course of the sampling period.
This is a noteworthy drawback especially considering that one of the application advantages of the composite sampler over process analysis and batch analysis is that composite sampling does not require an on-site operator and is practical at small or physically remote process locations. These advantages are severely compromised if an operator is required to make the number of observations necessary to ascertain that the sampling system is functioning properly.
Another drawback specific to the magnetic tracker type indicator is that the tracker can become decoupled from the piston due to fast piston movement or to accidental jarring. Once this decoupling occurs it must be identified and corrected by an operator.
A constant goal of the modern chemical and petroleum industries has been to automate the measurement and control of the most vital processes. The emphasis from the measurement side has been to develop a network of distributed transducers that determine the physical properties or quantities of the processes and transmit a proportional electronic signal to a central control computer. Reliable electronic transmitters have been developed for most of the basic operating properties of modern petroleum processes: temperature, pressure, density, flow, level, and even for some that are more complex such as viscosity, chemical composition, moisture and H.sub.2 S content and heating value.
One of the critical operating variables that is still only available from an indicator and that is not now electronically transmittable is the operation and "state of fullness" of the constant pressure sample vessel.
To overcome these shortcomings of the prior art, the present invention is provided to electronically sense the position of the piston and to transmit an electronic signal that is proportional to that position. The electronic signal can then be monitored by the typical programmable process controller, telemetering station, central control computer, or data acquisition system. The fact that an electronic signal is able to be transmitted eliminates the need for visual inspection and/or an operator.
The present invention is developed to take advantage of the prior art by making good use of the fact that almost all of the existing constant pressure cylinders are already fitted with magnets in order to operate the flag or tracker indicating systems. This invention is provided to directly sense the magnetic fields of the existing piston magnets in such a way that it can be added to an existing sample vessel without any necessary retrofitting and without interferring with the well-established indicator systems. This invention is operable by simply securing it within close proximity to the outside of the sample vessel in a generally parallel orientation and supplying it with the connections to a power supply and for a signal output; it requires no direct connection or even contact with the sample vessel or other sampler components.
The fact that this invention does not require direct contact or modification of the sample vessel is important because at the end of any one particular sampling period the full vessel is removed for analysis and replaced by another, empty vessel. The "no-contact" operation of this invention greatly simplifies the exchange process and again saves operator time.
Because of the variety of sample vessels used in industry, the second sample vessel, in almost every exchange, will have a different magnetic field from the first. This invention accommodates a wide variety of different magnetic fields and still produces a precise and accurate output signal.
The problem of variety in the magnetic field is exacerbated by the fact that the magnets are typically of large physical size but of only medium magnetic concentration, due to the variety of outer coverings that are used to protect the vessel and by the thickness of the cylinder wall.
Thus, the magnetic fields are typically large in size, low in strength, irregular in pattern and significantly isolated from any type of sensor system. This means that transmitters or transmitter systems that are simple and straight-forward or that are already commercially available are not very useful; a new, innovative transmitter, with a unique sensor array network and unique logic is required in order to have solid-state reliability in a reasonably accurate position transmitter.
Electronic transmitting means can be divided into two major groups according to the predominant signal circuitry; digital or analog. A digital device consists of electronic logic gates whose outputs can change only between a limited number of voltages and that transmits information in discrete form. Specifically, a simple binary digital signal has only two states that are defined by two voltage levels and thus a single data line can only transmit either a 0 or a 1 with in-between levels of voltage rejected as invalid transitional data. On the other hand, an analog device consists of electronic control circuits whose output varies smoothly and continuously in amplitude through a range of voltages or currents and that transmits information of infinite resolution over only one or two data lines.
Many common analog transmitters are designed around an electrical resistance network whose resistance value is determined by the value of the physical property in question. As an example, in a fluid pressure transmitter the resistance value of a strain gauge network is determined by the amount of strain exerted on a diaphragm by the pressure of the fluid acting on that diaphragm. In simple analog position transmitters, based on the use of a potentiometer, the resistance value of the electrical network is held constant while a moveable contact point that follows the position of the object in question divides the network proportionally and is used as the principal raw data line.
The most common digital transmitter with a single data output line is a switch; a device that changes output states when a physical property reaches predetermined values. The switch circuitry routes the analog output of a simple transducer directly into a comparator whose output switches to a high state (1) when the transducer's output rises above the set point, and to a low state (0) when the transducer's output falls below the reset point. Switch circuitry is commonplace and has been commercially developed to be sensitive to any number of physical properties and events including pressure, temperature, light, movement, acceleration, force, magnetic field, and the presence of inductive cores or capacitive plates. Switch outputs inherently lack resolution because each output can only transmit whether the physical property is above or below the setpoints.
Digital transmitters often attain a high degree of resolution with a single switch by monitoring the output of the switch to determine how many switching events occur during a specified length of time. Simple switch-based transmitters that utilize the concepts of time and count have been most successfully developed for measurement of flow, speed, and other time dependent processes.
Each major group, analog or digital, has advantages over the other. The signal from a digital device is usually more precise, independent of drifting power levels, and does not require amplification but, because of the binary nature of digital circuits, a digital signal of moderate resolution will consist of at least six to eight places transmitted over that number of parallel data lines. The signal from an analog device is easily transmitted over a single line but it generally requires amplification and thus increased power consumption, and it's accuracy is dependent on changes in power levels, temperature and electrical noise. A digital device usually consumes less power than an analog device, but typically requires more components in the form of complex gated circuitry and a more complex time-multiplexed logic scheme.
A simple transmitter is usually designed around only analog or digital circuits and therefore inherits the disadvantages of that circuit group. Of course, more complex "smart" microprocessor-based transmitters and complex computer interface systems combine the advantages of both analog and digital logic by generating raw analog data from a transducer and converting this data to digital form for processing, and then either reconverting the processed digital data to an analog signal, and transmitting the analog signal, or transmitting the digital data using the modulation techniques of a telephone modem or a conventional serial protocol such as RS-232. But accurate converter schemes are complex and expensive and consume more power than is practical, while modulation circuits are not only very complex, expensive and impractical but typically require either additional control lines (synchronous) or a third digital state defined as "no signal" (asynchronous).
The present invention combines the beneficial digital characteristics of high precision, high noise immunity and drift-free accuracy with the beneficial analog characteristics of simple, straight-forward logic, relatively few components, and one data line, while retaining the characteristics that are desired in a marketable position transmitter; low cost, easy installation, low power consumption, moderate resolution, and solid-state reliability.
These goals are achieved principally by taking advantage of the industrial process's requirements for response time. Industrial processes such as sampling fullness, level, and steady flow typically register significant change in terms of hours or days so that a transmitter with a response time of several minutes is more than adequate for maintaining the data as significant.
This response time of several minutes is used only as an example of the upper limits of response time; the present invention operates quite efficiently with a response time of tenth seconds. Note also, that although this sounds significantly slower than microprocessors operating at speeds of 10 MHz or 10 million steps per second, the overall response times for significant data updates in such microprocessor systems typically ranges from one to hundreds of milliseconds. Thus the response time of the present invention running at a fast speed is comparable to a slow microprocessor-controlled analog to digital conversion scheme.
The typical microprocessor based device exploits this conversion time to perform thousands of very fast operations while the present invention performs less than a hundred relatively slow operations to achieve a comparable response time. The typical microprocessor transmitting device consumes large amounts of power and time for controlling the excitation of the analog transducer, on converting and mathematically manipulating the data, and on synchronizing the data for transmission via a standard protocol. Only a small amount of time is consumed on performing the essential steps of monitoring the data and transmitting it. The present invention significantly reduces the number of operations by eliminating the need for the support operations described above, by retaining only the essential operations of sampling and transmitting, and by performing both of these operations simultaneously.
The present invention combines the network concept of analog circuitry with the switch concept of digital circuitry in order to produce a position transmitter device consisting of a network of proximity switches, a digital controller circuit, and a physical exciter that is coupled to the position of the object being monitored. The proximity switches are also referred to as physicoelectric proximity switches to designate a switch whose electric properties are influenced by the presence of the physical exciter. The present invention further transmits a string of pulses which can be routed simultaneously to the data output circuit and to the controller circuit such that the number of pulses in the string is directly proportional to the position of the object being monitored.
In one embodiment of the present invention, the network or sensor array circuit is a group of from 16 to 256 magnetically operated digital proximity switches whose power input lines are connected in a matrix of rows and columns and controlled by a power multiplexing circuit and whose data output lines are connected to form a single common sensor array output line. The power multiplexing circuit is addressed in hexadecimal code sent from the controller and is active as long as data pulses are generated. The physical exciter is a magnet.
When the power multiplexer is active, it connects positive voltage to the positive power input lines of a row of magnetic switches and connects ground voltage to the negative power input lines of a column of magnetic switches. Only one magnetic switch is connected to both positive voltage and ground and thus only one magnetic switch will be electrically energized at a time; and that switch's position is defined by the particular row and column to which the switch belongs. The controller can electrically energize any one particular position along the network by addressing it's particular row and column.
The controller circuitry typically includes a pulse generator, an eight bit hexadecimal counter, signal conditioning for the sensor array output line, and digital logic gates for resetting the counter and activating the power multiplexer. The data output line is connected to the pulse generator through a simple logic gate that is part of the controller circuit. In simple operation, pulses are generated continuously and by default are considered control pulses. Pulses are treated as data pulses after a predetermined number of control pulses have been generated and counted, at which time the power multiplexer is activated and addressed with the row and column of the first position and the data output line begins transmitting data pulses. As each additional data pulse is generated the position address is advanced by one position until a switch is electrically energized that lies within a magnetic field of sufficient strength, at which time the common sensor array output switches from a high state to a low state, causing the controller logic circuits to deactivate the power multiplexer, reset the counter and return to generating control pulses. Thus the number of data pulses that is transmitted in any one string is directly proportional to the position of the magnetic field, and therefore to the position of the object being monitored.
The present invention is developed in the particular embodiment described above because it has two distinct advantages over more obvious embodiments and over the prior art; the proximity switches can be electronically connected as a matrix, thus substantially reducing the number of connections and logic gates that are necessary if a large number of switches are used and one and only one proximity switch is electrically energized at any instant in time, thus substantially reducing the amount of energy that would normally be consumed if a large number of switches are used.
It is important to note that each proximity switch represents a specific position along the intended path of the object and, in turn, each position that is resolved must be represented by a switch. Thus the number of positions that can be resolved is directly equal to the number of switches in the sensor array network. Even a moderate resolution of 2% requires 50 switches to be used in a single sensor array.
The prior art would suggest that common power connections be made between all the proximity switches, that the power be continuously applied to all switches and that each output be separately monitored to determine the location of the magnetic field.
Considering for example, a sensor array of moderate resolution that consists of 64 magnetic proximity switches each of which has three connections, two for power and one output and each of which consumes 15 milliamperes of current at 5 volts dc; since a one connection output does not fit the criteria for a matrix, the prior art would require 64 logic gates just to monitor the outputs of each switch and would consume 960 mA of current or roughly 5 watts of power. The power consumption alone would make it completely useless in the kind of remote locations for which the present invention is developed. Even in industrial applications where power is not a supply problem, the typical transmitter consumes less than 50 mA of current and support devices such as data acquisition systems, supply wiring and communications equipment would have to be modified to handle this level of current. For the same resolution and with the same 64 magnetic switches, the present invention would require only 16 logic gates arranged in an 8.times.8 matrix, and would consume only 15 mA of current or 75 milliwatts of power; this is one-fourth the number of gates, 1/64 the amount of power and a substantial improvement over the prior art.